As mentioned at the beginning of this tutorial, Ford has made it easy to test the fuel pump with a fuel pressure gauge... since you'll find a Schrader valve on the fuel injector rail that you can connect a fuel pressure gauge to (see photos in the image viewer).
Using a fuel pressure gauge is one of the most accurate ways to make sure that enough fuel is reaching the fuel injectors. If you do own a fuel pressure gauge, this is the test for you.
If you don't own one, you can run down to your local Auto Zone or O'Reilly auto parts store and borrow one from them (for a small cash deposit, which you'll get back when you return the tool).
If you're interesting in buying one, take a look at my recommendations here: Which and Where to Buy a Fuel Pressure Gauge.
OK, let's get started with this test:
Connect the fuel pressure gauge to the Schrader valve on the fuel rail.
When ready, ask your helper to crank the engine while you observe the fuel pressure tester's gauge.
Your fuel pressure gauge will register one of the two following results:
1.) The fuel pressure gauge will register 35 PSI, or
2.) The fuel pressure gauge will register 0 PSI.
OK, now that the testing part is done... let's take a look at what your results mean:
CASE 1: If the fuel pressure gauge registered 0 PSI This confirms that the cause of your 4.0L Ford Explorer (Ranger, Aerostar, Mountaineer) is caused by a lack of fuel.
Now, I usually take two more precautions, before condemning the fuel pump as BAD... and this is to check:
To check that the fuel pump is getting power (after the inertia switch has been checked) you'll need to attach a multimeter in Volts DC mode to the wire that supplies this voltage to the fuel pump and while a helper cranks the engine... verify that the fuel pump is getting this power. If the 12 Volts are being supplied to the fuel pump... you have now 100% verified that the fuel pump is fried and needs to be replaced.
CASE 2: If the fuel pressure gauge registered 35 PSI: This fuel pressure gauge result let's you know that the fuel pump is working and delivering enough fuel to the fuel injectors. The reason your 4.0L Ford vehicle is not starting is due to another reason. The fuel pump is OK.
I suggest you take a look at the following tutorial: How to Troubleshoot a No Start (Ford 4.0L), to see more testing options.
Your 4.0L Ford Explorer (Ranger, Aerostar, or Mercury Mountaineer) comes equipped with a fuel pump inertia switch, whose job is to cut power (or ground) to the fuel pump in case of an impact... which causes the engine to stall (if it was running) or for the engine to crank but not start due to a lack of fuel.
Although it's designed to activate during a severe impact cause by an automobile accident... sometimes it gets activated by less than severe impacts (and this happens a lot).
This means that it's always a good idea to check and reset the inertia switch whenever testing the fuel pump. On the majority of the vehicles covered by this tutorial, the inertia switch is located below the right side of the instrument panel, behind the trim panel.
Resetting the fuel pump inertia switch simply involves pushing down on the button located on top of the inertia switch till it bottoms out. If the inertia switch has been jolted into shutting off the fuel pump... you'll feel a definite ‘click’ as you depress the button.
Owning a fuel pressure gauge is now becoming a must. All vehicles now use an electrical fuel pump to deliver fuel (under high pressure) to the fuel injectors and the best way to diagnose this fuel pump is with a fuel pressure gauge.
As mentioned earlier (in fuel pump test 2), in all of the Ford 4.0L V6 equipped vehicles... Ford was kind enough to put a Schrader valve on the fuel injector rail where you can tap into the fuel system and check its performance.
A fuel pressure gauge can cost you as little as $40 (US) or as much as $200 (US). The price difference depends on what type of fuel pressure gauge set you buy... this means either buying a Non-Professional Technician grade tool or a professional technician grade too. Whichever one you buy, it's an investment that will pay for itself many times over.
In the next page, I'm going to review two different fuel pressure gauge sets that will shed some light into which one is the one you may want/need to buy.
Alright, now, which one and where to buy it in the next page...
“For instance, on the planet Earth, man had always assumed that he was more
intelligent than dolphins because he had achieved so much —the wheel,
New York, wars and so on —whilst all the dolphins had ever done was
muck about in the water having a good time. But conversely,
the dolphins had always believed that they were far more
intelligent than man —for precisely the same reasons!”
The Hitchhiker's Guide to the Galaxy